Clinical evaluation of the Lack coaxial breathing circuit in small animal anaesthesia

Comparison of the Bain system and Uniflow universal anaesthetic breathing systems in spontaneously breathing young pigs

OBJECTIVE

To compare minimum fresh gas flow (V(min)) requirements and respiratory resistance in the Uniflow and Bain anaesthetic breathing systems used in the Mapleson D mode. Animals Seven pigs, aged 8-12 weeks, anaesthetized for ophthalmic surgery.

MATERIALS AND METHODS

Anaesthesia was maintained with halothane delivered in oxygen using a (Mapleson D) Bain breathing system. The V(min) that prevented re-breathing was found, and peak inspiratory (PIP) and peak expiratory (PEP) pressures measured. The fresh gas flow (V(f)) was then increased to V(min) + 50%, then V(min) + 100%, and respiratory pressures re-measured. A heat and moisture exchanger (HME) was inserted at the endotracheal tube and the procedure repeated. The breathing system was then exchanged for a Uniflow and the protocol repeated. After final disconnection from the breathing system, the animals' peak inspiratory and expiratory flows, tidal, and minute volumes (Vm) were measured over five respiratory cycles.

RESULTS

The V(min) (L minute(-1); mL kg(-1) minute(-1)) required to prevent rebreathing in the Uniflow system [8.1(mean) +/-1.7 (SD); 332 +/- 94] was significantly greater than the Bain system (6.5 +/- 1.1; 256 +/- 64). At V(min), PEP with the Uniflow (3.5 +/- 0.1 cm H(2)O) was significantly higher than the Bain system (2 +/- 0.7 cm H(2)O), but PIP values did not differ (Uniflow -0.6 +/- 2.1 cm H(2)O; Bain system -0.2 +/- 0.6 cm H(2)O). With both systems, PEP increased significantly (p < 0.001) with each increase in V(f): Uniflow system 4.2 +/- 0.4 (V(min) + 50%) and 5.5 +/- 0.5 cm H(2)O (V(min) + 100%); Bain system 2.8 +/- 0.7 (V(min) + 50%) and 3.5 +/- 0.7 cm H(2)O (V(min) + 100%). Insertion of the HME did not alter pressures. The mean tidal volume was 6.4 +/- 1.6 mL kg(-1); mean Vm was 184.9 +/- 69.8 mL kg(-1) and mean respiratory rate was 28 +/- 5 breaths minute(-1). In one pig breathing with the Uniflow system PEP rose sharply; respiratory and heart rates increased, and ventricular dysrhythmias occurred. When the system was changed and V(f) reduced, physiological variables became normal.

CONCLUSION

The study discredited the hypothesis that the two breathing systems behave similarly. Values for V(min) and PEP were higher with the Uniflow system. Increasing V(f) increased PEP with both systems. Insertion of an HME did not affect respiratory pressures.

CLINICAL RELEVANCE

The Uniflow used in Mapleson D mode is not suitable for anaesthesia in young spontaneously breathing pigs.

Clinical evaluation of a valveless non-absorber breathing system in spontaneously breathing canine patients

A valveless non-absorber breathing system novel to veterinary anaesthesia is described. The performance of this system was evaluated in 35 anaesthetised spontaneously breathing dogs weighing between 2.1 and 56 kg. Fresh gas flows were reduced incrementally until rebreathing (defined as an increase in end-inspired carbon dioxide tension above 0.2 per cent) started to occur, as measured by capnography. A significant relationship (P < 0.0001) between critical fresh gas flow and bodyweight was determined, and a mean critical fresh gas flow rate of 145 +/- 21 ml/kg/minute was derived for 15 dogs weighing 10 kg or less (mean 6.7 +/- 2.6 kg) and one of 98 +/- 16 ml/kg/minute for the remaining 20 dogs weighing 11 kg or greater (mean 30.2 +/- 13.9 kg). The fresh gas requirements for each group were found to differ significantly (P < 0.0001), although the correlation between critical fresh gas flow and bodyweight was not significant (P = 0.054) in the dogs weighing 10 kg or less. It is suggested that the system may prove an economical and useful addition to the breathing systems currently used in canine anaesthesia.